Russian Foreign Minister Sergei Lavrov has said Moscow is studying proposed changes to the U.S. missile defense program, but still wants guarantees that the system would not be used against Russia.

"We are now studying the proposals we received from the U.S. side recently on the further development of our dialogue on missile defense," he told reporters at NATO headquarters in Brussels

Lavrov met with NATO foreign ministers in Brussels on April 23, and also held bilateral talks with U.S. Secretary of State John Kerry.

Also in Brussels, called for Russia "to embrace the constructive approach to missile defense."

The Pentagon said last month that it would station additional missile interceptors in Alaska in response to North Korean threats and at the same time forgo a new type of interceptor that could have been deployed in Europe and which Russia believed could be used to shoot down its strategic missiles.

The NATO ministers also discussed the situation in Syria, and NATO Secretary-General Rasmussen said the alliance is "extremely concerned" about the possible use of chemical weapons there.

Lavrov said any reports that chemical weapons have been used in Syria should be investigated by experts on the ground.

The government and rebels in Syria have accused each other of using chemical weapons, and Lavrov said attempts to politicize a UN probe were unacceptable.

Kerry said Israeli Prime Minister Benjamin Netanyahu in a telephone conversation on April 23 had not been able to confirm comments by senior Israeli army officials claiming that Syrian government forces had used chemical weapons against rebels.

Washington has said that the use of chemical weapons by government forces would be crossing a red line in the Syrian conflict, which would require action by the international community.